Traditional brake systems use hydraulic pressure to generate the force necessary to slow down and stop cars and trucks. These systems are operated by a brake pedal that activates a master cylinder. When the master cylinder is activated, it generates hydraulic pressure in the brake lines. That pressure subsequently acts on the wheel cylinders present in each wheel, which either pinch a rotor between brake pads or press brake shoes outwards into a drum.
Modern hydraulic brake systems have increased complexity, but still work on the same general principle. Hydraulic or vacuum brake boosters reduce the amount of force the driver has to apply, and technologies such as anti-lock brakes and fraction control systems are capable of automatically activating or releasing the brakes.
More recent developments have resulted in brake-by-wire systems in which there is not a direct mechanical connection between the brake pedal and the wheel cylinder during normal operation. These systems still include hydraulic systems, but the driver does not directly activate the wheel cylinders by pressing on the brake pedal. Instead, the wheel cylinders are activated by an electric motor or pump that is regulated by a control unit. If normal operation is not possible, a failsafe mode of operation may provide a direct mechanical connection between the brake pedal and the wheel cylinder.